<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>watch</title>
</head>

<body>
    <div id="app">
        <input type="text" v-model="message" />
    </div>

    <script src="./node_modules/vue/dist/vue.js"></script>
    <script>
        var app = new Vue({
            el: '#app',
            data: {
                message: ''
            },
            watch: {
                message(newValue, oldValue) {
                    console.log(newValue + "----" + oldValue);
                }
            }
        });
    </script>
</body>

</html>